1. Field
Disclosed herein is a method for the treatment of material containing at least one valuable metal and arsenic to form a valuable metal-depleted scorodite sediment and a pure aqueous solution to be removed from the process. According to the method, the valuable metals are first removed from the material to be treated and then arsenic precipitation from the solution is performed in two stages. The aim is to use the method to obtain as low a valuable metal content as possible in the scorodite sediment that will be formed. Likewise, the arsenic and valuable metal content of the aqueous solution that is formed during arsenic precipitation also remains so low that the water can be released into the environment.
2. Description of Related Art
Arsenic appears in nature in many different formations. Very commonly arsenic appears with iron and copper, but also with nickel, cobalt, gold and silver. Arsenic is also the most important impurity to remove during recovery of non-ferrous metals. During pyrometallurgical processes the majority of arsenic remains in the fly ash of the waste heat boiler and electric furnace. The utilisation of arsenic has not grown in relation to its recovery, so the majority of arsenic has to be stored in the form of waste. Since arsenic and its compounds are toxic, they must be turned into as poorly soluble a form as possible before they are removed from the process. The less soluble arsenic compounds in the neutral pH zone are for instance zinc, copper and lead arsenates, but the binding of arsenic to these valuable metals is not under serious consideration, specifically because of the valuable metal content that remains in the waste. One current arsenic precipitation method that is frequently used is to precipitate arsenic with iron as ferric arsenate, which is fairly insoluble. In particular, the crystalline form of ferric arsenate, scorodite, FeAsO4.2H2O, is less soluble than its other form, amorphous ferric arsenate. One arsenic recovery method is described in CA patent application 2384664, which presents a method for the recovery of arsenic from an acidic solution that also contains copper and divalent and trivalent iron. Arsenic precipitation is performed in one stage, wherein the stage comprises several stirred tank reactors into which air is passed. The temperature of the reactors is held in the range of 60-100° C. to prevent the co-precipitation of copper. In order to precipitate the ferric arsenate, a neutralizing agent is fed into the reactors, helping to maintain the pH value between 1.5-1.9. The precipitated ferric arsenate is recycled to the first reactor and ferric arsenate compounds are fed into the solution as seeds. Arsenic recovery is connected to sulphidic concentrate leaching, which occurs by means of trivalent iron. The solution from concentrate leaching is routed to the arsenic removal described above, and the solution exiting arsenic removal is routed in turn to copper extraction.
U.S. Pat. No. 6,406,676 describes a method for removing arsenic and iron from an acidic solution that is generated in the hydrometallurgical treatment of concentrate. Arsenic and iron precipitation are performed in two steps, where the pH is kept in the range of 2.2-2.8 in the first precipitation step and between 3.0-4.5 in the second step. Lime is added to both precipitation steps and in addition air is injected in the second step. Each step produces its own iron-arsenic residue, and the residue from the second step is recycled to the first step where any unreacted lime can be exploited in the first stage. The residue from the second step can also be recycled to the beginning of the same step to improve the crystallisation of the residue. According to the example, the method is applicable for a zinc-containing solution and it is stated that zinc is not precipitated with the iron and arsenic, but can be recovered after this treatment.
The article by Wang, Q. et al entitled “Arsenic Fixation in Metallurgical Plant Effluents in the Form of Crystalline Scorodite via a Non-Autoclave Oxidation-Precipitation Process”, Society for Mining Metallurgy and Exploration, Inc, 2000, describes a method for removing arsenic from fly ash, in which arsenic is recovered as scorodite. The first treatment stage of the arsenic-containing material is the oxidation of trivalent arsenic (As(III)) into pentavalent arsenic (As(V)) with a gas containing sulphur dioxide and oxygen in oxidising conditions, in which arsenic does not precipitate. After this, arsenic is precipitated in atmospheric conditions, in which the Fe(III)/As(V) mole ratio is specified as 1. Precipitation is carried out either in one or several stages, but precipitation as scorodite demands the over-saturation of the solution, which is achieved by recycling scorodite crystals to the first precipitation reactors and simultaneously neutralising the suspension. A beneficial pH range is around 1-2 and this is maintained by feeding a suitable neutralising agent into the precipitation stage. In these conditions, arsenic can be precipitated to the level of 0.5 g/l. The final arsenic removal to a level below 0.1 mg/l is done by means of a second purification stage, in which the iron and arsenic Fe(III)/As(V) mole ratio is adjusted to a value in a range of 3-5 and the pH to a value between 3.5-5. The amorphous precipitate generated in this stage is routed back to the first precipitation stage, where it dissolves and precipitates again as scorodite. It is stated in the article that if valuable metals are present in the solution, they can be recovered after arsenic precipitation.
The tests described in the article mentioned above give a good understanding of arsenic precipitation, but in all the tests carried out, arsenic precipitation was done first and recovery of valuable metals afterwards. The disadvantage of these methods is that water-soluble valuable metals originating from an alkaline solution remain in the ferric arsenate residue precipitated from the solution containing valuable metals, and cannot be recovered even after thorough washing.